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Pedro Falcato's optimization [1] for checking sealed VMAs, which replaces
the can_modify_mm() function with an in-loop check, necessitates an update
to the mseal.rst documentation to reflect this change.

Furthermore, the document has received offline comments regarding the code
sample and suggestions for sentence clarification to enhance reader
comprehension.

[1] https://lore.kernel.org/linux-mm/20240817-mseal-depessimize-v3-0-d8d2e037df30@gmail.com/

Update doc after in-loop change: mprotect/madvise can have
partially updated and munmap is atomic.

Fix indentation and clarify some sections to improve readability.

Link: https://lkml.kernel.org/r/20241008040942.1478931-2-jeffxu@chromium.org
Fixes: df2a7df ("mm/munmap: replace can_modify_mm with can_modify_vma")
Fixes: 4a2dd02 ("mm/mprotect: replace can_modify_mm with can_modify_vma")
Fixes: 3807567 ("mm/mremap: replace can_modify_mm with can_modify_vma")
Fixes: 23c57d1 ("mseal: replace can_modify_mm_madv with a vma variant")
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Elliott Hughes <enh@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Stephen Röttger <sroettger@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: "Theo de Raadt" <deraadt@openbsd.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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307 changes: 148 additions & 159 deletions Documentation/userspace-api/mseal.rst
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Expand Up @@ -23,177 +23,166 @@ applications can additionally seal security critical data at runtime.
A similar feature already exists in the XNU kernel with the
VM_FLAGS_PERMANENT flag [1] and on OpenBSD with the mimmutable syscall [2].

User API
========
mseal()
-----------
The mseal() syscall has the following signature:

``int mseal(void addr, size_t len, unsigned long flags)``

**addr/len**: virtual memory address range.

The address range set by ``addr``/``len`` must meet:
- The start address must be in an allocated VMA.
- The start address must be page aligned.
- The end address (``addr`` + ``len``) must be in an allocated VMA.
- no gap (unallocated memory) between start and end address.

The ``len`` will be paged aligned implicitly by the kernel.

**flags**: reserved for future use.

**return values**:

- ``0``: Success.

- ``-EINVAL``:
- Invalid input ``flags``.
- The start address (``addr``) is not page aligned.
- Address range (``addr`` + ``len``) overflow.

- ``-ENOMEM``:
- The start address (``addr``) is not allocated.
- The end address (``addr`` + ``len``) is not allocated.
- A gap (unallocated memory) between start and end address.

- ``-EPERM``:
- sealing is supported only on 64-bit CPUs, 32-bit is not supported.

- For above error cases, users can expect the given memory range is
unmodified, i.e. no partial update.

- There might be other internal errors/cases not listed here, e.g.
error during merging/splitting VMAs, or the process reaching the max
number of supported VMAs. In those cases, partial updates to the given
memory range could happen. However, those cases should be rare.

**Blocked operations after sealing**:
Unmapping, moving to another location, and shrinking the size,
via munmap() and mremap(), can leave an empty space, therefore
can be replaced with a VMA with a new set of attributes.

Moving or expanding a different VMA into the current location,
via mremap().

Modifying a VMA via mmap(MAP_FIXED).

Size expansion, via mremap(), does not appear to pose any
specific risks to sealed VMAs. It is included anyway because
the use case is unclear. In any case, users can rely on
merging to expand a sealed VMA.

mprotect() and pkey_mprotect().

Some destructive madvice() behaviors (e.g. MADV_DONTNEED)
for anonymous memory, when users don't have write permission to the
memory. Those behaviors can alter region contents by discarding pages,
effectively a memset(0) for anonymous memory.

Kernel will return -EPERM for blocked operations.

For blocked operations, one can expect the given address is unmodified,
i.e. no partial update. Note, this is different from existing mm
system call behaviors, where partial updates are made till an error is
found and returned to userspace. To give an example:

Assume following code sequence:

- ptr = mmap(null, 8192, PROT_NONE);
- munmap(ptr + 4096, 4096);
- ret1 = mprotect(ptr, 8192, PROT_READ);
- mseal(ptr, 4096);
- ret2 = mprotect(ptr, 8192, PROT_NONE);

ret1 will be -ENOMEM, the page from ptr is updated to PROT_READ.

ret2 will be -EPERM, the page remains to be PROT_READ.

**Note**:

- mseal() only works on 64-bit CPUs, not 32-bit CPU.

- users can call mseal() multiple times, mseal() on an already sealed memory
is a no-action (not error).

- munseal() is not supported.

Use cases:
==========
SYSCALL
=======
mseal syscall signature
-----------------------
``int mseal(void \* addr, size_t len, unsigned long flags)``

**addr**/**len**: virtual memory address range.
The address range set by **addr**/**len** must meet:
- The start address must be in an allocated VMA.
- The start address must be page aligned.
- The end address (**addr** + **len**) must be in an allocated VMA.
- no gap (unallocated memory) between start and end address.

The ``len`` will be paged aligned implicitly by the kernel.

**flags**: reserved for future use.

**Return values**:
- **0**: Success.
- **-EINVAL**:
* Invalid input ``flags``.
* The start address (``addr``) is not page aligned.
* Address range (``addr`` + ``len``) overflow.
- **-ENOMEM**:
* The start address (``addr``) is not allocated.
* The end address (``addr`` + ``len``) is not allocated.
* A gap (unallocated memory) between start and end address.
- **-EPERM**:
* sealing is supported only on 64-bit CPUs, 32-bit is not supported.

**Note about error return**:
- For above error cases, users can expect the given memory range is
unmodified, i.e. no partial update.
- There might be other internal errors/cases not listed here, e.g.
error during merging/splitting VMAs, or the process reaching the maximum
number of supported VMAs. In those cases, partial updates to the given
memory range could happen. However, those cases should be rare.

**Architecture support**:
mseal only works on 64-bit CPUs, not 32-bit CPUs.

**Idempotent**:
users can call mseal multiple times. mseal on an already sealed memory
is a no-action (not error).

**no munseal**
Once mapping is sealed, it can't be unsealed. The kernel should never
have munseal, this is consistent with other sealing feature, e.g.
F_SEAL_SEAL for file.

Blocked mm syscall for sealed mapping
-------------------------------------
It might be important to note: **once the mapping is sealed, it will
stay in the process's memory until the process terminates**.

Example::

*ptr = mmap(0, 4096, PROT_READ, MAP_ANONYMOUS | MAP_PRIVATE, 0, 0);
rc = mseal(ptr, 4096, 0);
/* munmap will fail */
rc = munmap(ptr, 4096);
assert(rc < 0);

Blocked mm syscall:
- munmap
- mmap
- mremap
- mprotect and pkey_mprotect
- some destructive madvise behaviors: MADV_DONTNEED, MADV_FREE,
MADV_DONTNEED_LOCKED, MADV_FREE, MADV_DONTFORK, MADV_WIPEONFORK

The first set of syscalls to block is munmap, mremap, mmap. They can
either leave an empty space in the address space, therefore allowing
replacement with a new mapping with new set of attributes, or can
overwrite the existing mapping with another mapping.

mprotect and pkey_mprotect are blocked because they changes the
protection bits (RWX) of the mapping.

Certain destructive madvise behaviors, specifically MADV_DONTNEED,
MADV_FREE, MADV_DONTNEED_LOCKED, and MADV_WIPEONFORK, can introduce
risks when applied to anonymous memory by threads lacking write
permissions. Consequently, these operations are prohibited under such
conditions. The aforementioned behaviors have the potential to modify
region contents by discarding pages, effectively performing a memset(0)
operation on the anonymous memory.

Kernel will return -EPERM for blocked syscalls.

When blocked syscall return -EPERM due to sealing, the memory regions may
or may not be changed, depends on the syscall being blocked:

- munmap: munmap is atomic. If one of VMAs in the given range is
sealed, none of VMAs are updated.
- mprotect, pkey_mprotect, madvise: partial update might happen, e.g.
when mprotect over multiple VMAs, mprotect might update the beginning
VMAs before reaching the sealed VMA and return -EPERM.
- mmap and mremap: undefined behavior.

Use cases
=========
- glibc:
The dynamic linker, during loading ELF executables, can apply sealing to
non-writable memory segments.

- Chrome browser: protect some security sensitive data-structures.
mapping segments.

Notes on which memory to seal:
==============================
- Chrome browser: protect some security sensitive data structures.

It might be important to note that sealing changes the lifetime of a mapping,
i.e. the sealed mapping won’t be unmapped till the process terminates or the
exec system call is invoked. Applications can apply sealing to any virtual
memory region from userspace, but it is crucial to thoroughly analyze the
mapping's lifetime prior to apply the sealing.
When not to use mseal
=====================
Applications can apply sealing to any virtual memory region from userspace,
but it is *crucial to thoroughly analyze the mapping's lifetime* prior to
apply the sealing. This is because the sealed mapping *won’t be unmapped*
until the process terminates or the exec system call is invoked.

For example:
- aio/shm
aio/shm can call mmap and munmap on behalf of userspace, e.g.
ksys_shmdt() in shm.c. The lifetimes of those mapping are not tied to
the lifetime of the process. If those memories are sealed from userspace,
then munmap will fail, causing leaks in VMA address space during the
lifetime of the process.

- ptr allocated by malloc (heap)
Don't use mseal on the memory ptr return from malloc().
malloc() is implemented by allocator, e.g. by glibc. Heap manager might
allocate a ptr from brk or mapping created by mmap.
If an app calls mseal on a ptr returned from malloc(), this can affect
the heap manager's ability to manage the mappings; the outcome is
non-deterministic.

Example::

ptr = malloc(size);
/* don't call mseal on ptr return from malloc. */
mseal(ptr, size);
/* free will success, allocator can't shrink heap lower than ptr */
free(ptr);

mseal doesn't block
===================
In a nutshell, mseal blocks certain mm syscall from modifying some of VMA's
attributes, such as protection bits (RWX). Sealed mappings doesn't mean the
memory is immutable.

- aio/shm

aio/shm can call mmap()/munmap() on behalf of userspace, e.g. ksys_shmdt() in
shm.c. The lifetime of those mapping are not tied to the lifetime of the
process. If those memories are sealed from userspace, then munmap() will fail,
causing leaks in VMA address space during the lifetime of the process.

- Brk (heap)

Currently, userspace applications can seal parts of the heap by calling
malloc() and mseal().
let's assume following calls from user space:

- ptr = malloc(size);
- mprotect(ptr, size, RO);
- mseal(ptr, size);
- free(ptr);

Technically, before mseal() is added, the user can change the protection of
the heap by calling mprotect(RO). As long as the user changes the protection
back to RW before free(), the memory range can be reused.

Adding mseal() into the picture, however, the heap is then sealed partially,
the user can still free it, but the memory remains to be RO. If the address
is re-used by the heap manager for another malloc, the process might crash
soon after. Therefore, it is important not to apply sealing to any memory
that might get recycled.

Furthermore, even if the application never calls the free() for the ptr,
the heap manager may invoke the brk system call to shrink the size of the
heap. In the kernel, the brk-shrink will call munmap(). Consequently,
depending on the location of the ptr, the outcome of brk-shrink is
nondeterministic.


Additional notes:
=================
As Jann Horn pointed out in [3], there are still a few ways to write
to RO memory, which is, in a way, by design. Those cases are not covered
by mseal(). If applications want to block such cases, sandbox tools (such as
seccomp, LSM, etc) might be considered.
to RO memory, which is, in a way, by design. And those could be blocked
by different security measures.

Those cases are:

- Write to read-only memory through /proc/self/mem interface.
- Write to read-only memory through ptrace (such as PTRACE_POKETEXT).
- userfaultfd.
- Write to read-only memory through /proc/self/mem interface (FOLL_FORCE).
- Write to read-only memory through ptrace (such as PTRACE_POKETEXT).
- userfaultfd.

The idea that inspired this patch comes from Stephen Röttger’s work in V8
CFI [4]. Chrome browser in ChromeOS will be the first user of this API.

Reference:
==========
[1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274

[2] https://man.openbsd.org/mimmutable.2

[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@mail.gmail.com

[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc
Reference
=========
- [1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274
- [2] https://man.openbsd.org/mimmutable.2
- [3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@mail.gmail.com
- [4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc

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